CRYSTAL GROWTH & DESIGN
Parallel and Inclined (1D f 2D) Interlacing Modes in New Polyrotaxane Frameworks [M2(bix)3(SO4)2] [M ) Zn(II), Cd(II); Bix ) 1,4-Bis(imidazol-1-ylmethyl)benzene]
2005 VOL. 5, NO. 1 37-39
Lucia Carlucci, Gianfranco Ciani,* and Davide M. Proserpio Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Via G. Venezian 21, 20133 Milano, Italy Received June 27, 2004;
Revised Manuscript Received August 27, 2004
ABSTRACT: The reactions of MII sulfates (M ) Zn, Cd) with the flexible ligand 1,4-bis(imidazol-1-ylmethyl)benzene (bix) yield the novel interesting coordination networks [M2(bix)3(SO4)2], containing 1D polymeric motifs of alternating rings and rods, which show extended rotaxane-like mechanical links producing 2D sheets via unprecedented parallel (M ) Zn) or inclined (M ) Cd) interlacing modes of the chains. A variety of coordination polymers have been reported in recent years1 that are of great current interest not only for their potential properties as functional solid materials,2 but also for their intriguing architectures and topologies. Particular attention has been recently focused on systems consisting of entangled individual motifs, including relatively sophisticated families such as polycatenated arrays, Borromean architectures, polyrotaxane-like species, interlocked motifs of different topology and/or dimensionality, and interweaved chains and helixes, whose topological analysis is in progress.3 Polythreaded systems, which can be considered as extended periodic analogues of the molecular rotaxanes or pseudo-rotaxanes, are quite rare.3a The distinct motifs in these species are not catenated but entangled via rotaxane-like mechanical links that require the presence of closed loops as well as of elements that can thread the loops. These two moieties may belong to the same polymeric species, as in the case of 1D chains of alternating rings and rods (see I in Scheme 1). Two polyrotaxane motifs involving infinite interlaced polymers of type I have been as yet reported, resulting in (1D f 1D) and (1D f 2D) polythreaded arrays, which are schematically illustrated in Scheme 1 (II4 and III,5a respectively). Conformationally nonrigid ligands are more suited to produce these new classes of compounds. We are currently investigating the use of the flexible ligand 1,4-bis(imidazol1-ylmethyl)benzene (bix), together with different MSO4 salts, since this spacer has already proven a certain ability to give uncommon entanglements, including also two remarkable polyrotaxane polymers,5 thanks to its different conformations. We have already reported on two fascinating polymers: the 3D network [Co(bix)2(H2O)2](SO4)‚ 7H2O,6a containing ribbons of rings catenated to a 3D single frame, and the unusual 3D polycatenated species [Co(bix)2](SO4)‚7H2O,6b comprised of two interlocked sets of different 2D layers. We describe here the reactions of bix with Zn(II) and Cd(II) sulfates that afford two different 2D networks of composition [M2(bix)3(SO4)2] (1, M ) Zn; 2, M ) Cd). Both species contain chains of rings and rods (I, in Scheme 1) that are interlocked in an “inclined” fashion in compound 2, but, for the first time, in a “parallel” fashion in compound 1. Colorless crystals of compound 1, [Zn2(bix)3(SO4)2]‚8H2O, are obtained in good yield by slow diffusion of solutions of bix in acetone into water solutions of Zn(II) sulfate, with a metal-to-ligand molar ratio of 1:2. The bulk polycrystalline material obtained by mixing the reagents in the correct ratio (1:1.5) corresponds to almost pure 1, as evidenced by
Scheme 1
X-ray powder diffraction methods. On performing the slow diffusion with Cd(II) sulfate under the same reaction conditions adopted for Zn(II) sulfate, a mixture of two crystalline species is obtained, i.e., [Cd2(bix)3(SO4)2] (2) and [Cd2(bix)2(H2O)4(SO4)2]‚(bix) (3), (65 vs. 35% respectively). On the other hand, the bulk material isolated from mixing the reagents in the 1:1.5 ratio is pure 3, the kinetic product. The crystals are stable in air for long times and their structures have been characterized by single-crystal X-ray analyses.7 The structures of compounds 1 and 2 are both comprised of 1D chains of alternating rings and rods, as illustrated in Figure 1 (inside the rings: Zn‚‚‚Zn 11.17 Å, Cd‚‚‚Cd 10.29 Å; ring connections: Zn‚‚‚Zn 10.09 Å, Cd‚‚‚Cd 14.88 Å). In 1 the polymeric chains are highly undulated (Figure 1, top) and propagate in the same direction, i.e., along [1 0 1], with a period of 28.62 Å (two rings + two rods). The metal centers display distorted tetrahedral geometry with three imidazole groups and a terminally bonded monodentate sulfate (Zn-O 1.89, 1.95 Å, Zn-N 1.98-2.00 Å, N-Zn-N,O 99.4-117.0°). All the chains are interlaced with two adjacent ones, thus generating a 2D polyrotaxane layer (see Figure 2). It is worth noting that in each chain the rings are alternately threaded by rods belonging to the two nearest chains placed by opposite sides. This fascinating (1D f 2D) array shows a type of entanglement that is completely unprecedented in polythreaded polymeric species,3a and, by analogy with the
10.1021/cg049794+ CCC: $30.25 © 2005 American Chemical Society Published on Web 09/21/2004
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Communications
Figure 1. The 1D chains of rings and rods in 1 (top) and 2 (bottom).
Figure 3. The inclined interlacing of the chains in a polyrotaxane layer of 2. The cross-linking sulfates, giving a “self-threaded” sheet, are evidenced.
Figure 2. Parallel interlacing of the chains in a polyrotaxane layer of 1 (top) and a schematic view of the chain entanglement (bottom).
interlocking modes known for polycatenanes, we can use in this case the term “parallel” (see Figure 2, bottom) in contrast with the term “inclined” interlacing (see III in Scheme 1), like that previously found in [Ag2(bix)3](NO3)2.5a The 2D layers stack along the [5 0 2h ] crystallographic direction with an interlayer distance of 13.5 Å. Most of the clathrate water molecules are located in the interlayer regions (overall calculated free voids: 22.7% of the cell volume) and can be completely removed, without loss of crystallinity, upon heating the sample up to ca. 100 °C. Water molecules are regained upon exposure to water vapor overnight, but the successive XRPD spectra reveal some crystal modifications. In compound 2 the chains are much more stretched out (see Figure 1, bottom) than in 1, due to the different conformations of the bix ligands joining the rings (gauche in 1 vs. anti in 2). Two identical sets of chains, with a period of 23.82 Å (one ring + one rod), span two distinct directions of propagation ([1 1 1h ] and [1 h 1 1]), and are interlaced in the inclined fashion to give a 2D layer (see Figure 3).
Compound 2, despite the different components, is isomorphous with [Ag2(bix)3](NO3)2,5a and the layers are similar, except for the fact that the anions in the present case crosslink adjacent chains. This originates a sheet that could be defined as being “self-threaded”. The µ2-η2-sulfates complete the coordination sphere of the Cd(II) centers, which display a trigonal bipyramidal geometry distorted toward square pyramidal, with three equatorial N(imidazole) atoms and two axial O(sulfate) atoms [Cd-N 2.24-2.28 Å, Cd-O 2.26, 2.28 Å, N-Cd-N 113.0-132.3°, N-Cd-O 82.2-104.3°, O-Cd-O 156.2°]. The stacking of the layers occurs along the [1 0 1] direction with an interlayer distance of 9.40 Å. The packing is more efficient than in 1, with no free voids left. The different entanglements of the chains in 1 and 2, accompanied by different conformations of the ligands, are driven by factors difficult to be clearly established. Probably the greater dimensions of the Cd(II) ions favor a higher coordination number, inducing a bridging role for the sulfates and therefore a different organization of the whole array. Compound 3, [Cd2(bix)2(H2O)4(SO4)2]‚(bix), does not belong to the family of polyrotaxanes, and contains 1D simple -bix-Cd(H2O)2-bix-Cd(H2O)2- chains cross-linked by bridging µ2-η2-sulfates to give 2D (4,4) layers of rhombic meshes (see Figure 4). Interestingly, an additional uncoordinated ligand every two metals forms N‚‚‚H-O hydrogen bond bridges with two water molecules coordinated to two different Cd centers, and the Cd-H2O‚‚‚bix‚‚‚H2O-Cd bridges thread the rhombic meshes. Thermal analyses reveal that 3 loses the coordinated H2O on heating to ca. 100 °C. Further heating of the sample leads to an exother-
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Crystal Growth & Design, Vol. 5, No. 1, 2005 39
References
Figure 4. A 2D layer in compound 3. The uncoordinated bix ligands (in blue) are hydrogen bridged to coordinated water molecules (broken links) and thread one-half of the rhombic meshes.
mic phase change at ca. 250 °C, which corresponds to the complete transformation to 2, as shown by XRPD monitoring. Rotaxanes are intriguing species for their potential nanotechnological applications, as molecular machines and switches,8 as well as for the topological aspects related to their mechanical links.3a Polyrotaxanes, the extended periodic version of such motifs, are also expected to show interesting properties, and the possibility of different modes of entanglement observed here opens new perspectives in an almost completely unexplored area of chemical topology. Supporting Information Available: Crystallographic information files (CIF) are available free of charge via the Internet at http://pubs.acs.org.
(1) See for example: (a) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1461; (b) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2639; (c) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629; (d) Eddaoudi, M.; Moler, D. B.; Li, H. L.; Chen, B. L.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (2) Janiak C. Dalton Trans. 2003, 2781. (3) (a) Carlucci, L.; Ciani, G.; Proserpio, D. M. Coord. Chem. Rev. 2003, 246, 247; (b) Carlucci, L.; Ciani, G.; Proserpio, D. M. CrystEngComm 2003, 5, 269; (c) Batten, S. R. CrystEngComm 3 (Art. 18), 2001, 67. (4) (a) Kuehl, C. J.; Tabellion, F. M.; Arif, A. M.; Stang, P. J. Organomet. 2001, 20, 1956; (b) Fraser, C. S. A.; Jennings, M. C.; Puddephatt, R. J. Chem. Commun. 2001, 1310. (5) (a) Hoskins, B. F.; Robson, R.; Slizys, D. A. J. Am. Chem. Soc. 1997, 119, 2952; (b) Hoskins, B. F.; Robson, R.; Slizys, D. A. Angew. Chem., Int. Ed. Engl. 1997, 36, 2336. (6) (a) Carlucci, L.; Ciani, G.; Proserpio, D. M. Chem. Commun. 2004, 380; (b) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Spadacini, L. CrystEngComm 2004, 6, 96. (7) Crystal data for compound 1: C42H58N12O16S2Zn2, monoclinic, a ) 15.796(3), b ) 13.704(3), c ) 24.555(4) Å, β ) 92.420(5)°, U ) 5310.6(17) Å3, space group P21/n (no. 14), Z ) 4, µ(Mo-KR) ) 1.059 mm-1. Least-squares refinement based on 4003 reflections with I > 2σ(I) and 628 parameters led to final R1 ) 0.0884, wR2 ) 0.2376. 2 C21H21CdN6O4S, monoclinic, a ) 13.813(1), b ) 11.176(1), c ) 14.401(1) Å, β ) 96.37(1)°, U ) 2209.4(3) Å3, space group P21/n (no. 14), Z ) 4, µ(Mo-KR) ) 1.125 mm-1. Least-squares refinement based on 5580 reflections with I > 2σ(I) and 298 parameters led to final R1 ) 0.0234, wR2 ) 0.0595. 3 C21H25CdN6O6S, monoclinic, a ) 10.132(1), b ) 18.624(2), c ) 12.604(1) Å, β ) 100.30(1)°, U ) 2340.0(3) Å3, space group P21/c (no. 14), Z ) 4, µ(Mo-KR) ) 1.074 mm-1. Least-squares refinement based on 4679 reflections with I > 2σ(I) and 319 parameters led to final R1 ) 0.0286, wR2 ) 0.0670. (8) See for example: Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348.
CG049794+
